1. Field of the Invention
The present invention relates to an apparatus for continuous vacuum forming of a plasticized material in which a flexible patterned belt having apertures therein is continuously rotatable with a perforated flat support belt over a vacuum chamber to provide improved stability of the belts and improved vacuum characteristics between the belts and the material.
2. Description of the Prior Art
Prior art apparatus for continuous forming of molded plastic sheets have used a continuously moving conveyor belt where the sheet is vacuum formed. As described in U.S. Pat. No. 3,982,868, an extruder deposits a thermoplastic material to an endless conveyor belt mold. The conveyor belt mold is made of a flexible material contoured with the grooved design shown on the product. The conveyor belt mold has a plurality of holes therethrough to provide means for exerting a vacuum onto the thermoplastic material received by the belt. A vacuum box located beneath the conveyor belt mold provides means which exerts suction on the thermoplastic material to shape the material into the mold design of the surface.
U.S. Pat. No. 3,966,383 describes an apparatus for producing an embossed thermoplastic sheet in which an endless, seamless screen is used as the embossing surface. The screen is rotated by a plurality of rotatable rolls. The perforated screen is advanced at the same rate as a heated film. A vacuum assembly applies a vacuum to a part of the undersurface of the perforated screen to pull the heated film into contact with the upper surface of the screen to cause the film to assume the shape provided on the top surface of the screen or belt. The vacuum assembly includes a vacuum manifold connected to end plates. Seal rolls are rotatably supported which contact the underside of the screen. Two movable seal members are connected to the end plates adjacent the seal rolls for making sliding sealing contact with the manifold, seal rolls and the underside of the screen. The seal rolls can be constructed from a fluorine containing polymeric material such as Teflon.RTM.. The above described patents use of a single conveyor belt has the shortcoming of frequent breakage of the belt during application of the vacuum to the belt. Also, the use of a single conveyor belt has the disadvantage that the outside surface of the belt tends move faster than its inside surface creating creep of the belt when vacuum is applied to the belt.
U.S. Pat. No. 5,314,325, issued to one of the inventors of this disclosure, describes a continuous vacuum forming apparatus including a pair of flexible belts for facilitating the vacuum forming operation and providing a flat surface against which the vacuum can be performed. The upper belt is formed of a flexible rubber material and the lower belt is formed of an open weave metallic material to allow air to be drawn therethrough. A flexibly resilient pattern form including a plurality of apertures is applied to the upper belt. The woven support belt is formed of an open weave material including longitudinal and lateral sections impregnated with rubber material which is impervious to air flow. The sections form a plurality of rectangular vacuum sections through which air can pass to the open weave material. The sections concentrate the vacuum into bands or channels. The apertures in the pattern form are in fluid flow communication with the vacuum sections to draw the hot plasticized material onto the patterned form. This patent has the shortcoming that the use of a woven belt with impregnated sections creates a belt which is not completely flat resulting in loss of vacuum around the raised surfaces. Also, the use of broad channels for vacuum creates sections along the belt without vacuum.